In the related art, in a case where a part in motion is imaged by an X-ray computed tomography (CT) apparatus, artifacts are caused in an obtained tomographic image due to the motion. In order to reduce the artifacts, generally, measurement is performed in accordance with physiological activity by using a biosensor such as an electrocardiograph or a respiration sensor, and imaging is controlled or an image is processed by using an obtained measurement signal. For example, in imaging targeting the heart, an electrocardiographic synchronous reconstruction method is performed in which projection data with a phase in which motion is reduced is collected on the basis of electrocardiographic information measured by using an electrocardiograph, and a tomographic image is reconstructed. Consequently, it is possible to obtain an image of a relatively stationary heart.
PTL 1 discloses a dose modulation method in which a relatively high dose of X-rays is applied in a specific heartbeat phase in which motion is reduced, and a low dose of X-rays is applied in other heartbeat phases, and thus an amount of radiation exposure is reduced while an image with phases other than the specific heartbeat phase can also be created.
In the method disclosed in PTL 1, an image with a phase in which imaging is performed in a low dose (hereinafter, referred to as a low dose phase image) has more deteriorated image quality than an image with a phase in which imaging is performed in a high dose. Thus, for example, in a case where images with all phases are reconstructed in order to observe an object over time, image quality varies due to a difference in an X-ray dose depending on the phases. In order to compensate for such a variation in image quality, an image quality improvement process may be performed on a low dose phase image. The image quality improvement process is a technique in which, for example, projection data is corrected, an image is reconstructed through successive approximation by using the corrected projection data, and thus a high image quality is achieved. In this image quality improvement process, an operator may designate an intensity level of the image quality improvement process in relation to the extent of improvement of image quality.